Fuel cell separators, together with carrying out the roles of imparting electrical conductivity to each unit cell and of providing flow channels for the supply of fuel and air (oxygen) to the unit cells, also serve as boundary walls separating the unit cells. Characteristics required of a separator thus include high electrical conductivity, high impermeability to gases, chemical stability, heat resistance and hydrophilicity. Techniques known for increasing these characteristics include the methods disclosed in Patent Documents 1 to 5.
Patent Document 1, the object of which is to provide a method for producing a solid polymer fuel cell separator that can be stably used over a long period of time, discloses a method for producing a solid polymer fuel cell separator which is molded under heat and pressure from a composition containing a phenolic resin solution and graphite powder and the cured form of which has a saturation water uptake of 3% or less.
Patent Document 2, the object of which is to provide an excellent fuel cell separator that can be molded as a thin-walled separator, that has sufficient strength and flexibility, and moreover that has little variation in thickness even when it is of reduced thickness, discloses a fuel cell separator which is obtained by molding a composition containing a porous synthetic graphite material, an epoxy resin that includes a base resin and a curing agent, and an internal mold release agent, and which has an average thickness for the thin-walled regions of from 0.12 to 0.20 mm.
Patent Document 3, the object of which is to provide a resin composition for fuel cell separators that can reduce the fuel cell separator thickness, enhance the thickness accuracy, make the electrical conductivity more uniform and increase the mechanical strength, discloses a liquid resin composition for fuel cell separators which includes: (A) graphite particles; (B) an epoxy resin component made up of, as at least part of the epoxy resin within a thermosetting resin, an ortho-cresol novolak-type epoxy resin or an ortho-cresol novolak-type epoxy resin, and at least one resin selected from among bisphenol-type epoxy resins, biphenyl-type epoxy resins and phenol aralkyl-type epoxy resins having a biphenylene skeleton; (C) a curing agent consisting at least in part of a phenolic resin, and (D) a curing accelerator that consists at least in part of a substituted imidazole having a hydrocarbon group at the 2 position.
Patent Document 4, the object of which is to provide both a solid polymer fuel cell separator having excellent properties such as impermeability to gases, strength characteristics, electrical properties and mold release properties during molding, and also a method of manufacture thereof, discloses a fuel cell separator in which a phenolic resin containing at least 50% of a high para-novolak-type phenolic resin is used as an epoxy resin curing agent.
Patent Document 5, the object of which is to exhibit a high moisture resistance while maintaining such properties as a high glass transition temperature and a good continuous moldability, discloses a fuel cell separator in which 2,3-dihydro-1H-pyrrolo(1,2-a)benzimidazole is used as a curing accelerator.
Although the separator of Patent Document 1 has a low water uptake (saturation water uptake) when immersed for ten days in hot water at 80° C. of 0.4 to 0.6%, because this remains inadequate for sustained power generation by a fuel cell operating at generally from 60 to 80° C., drawbacks include decreased performance due to swelling of the separator from water absorption, and also cracking and breakage on account of non-uniform extension. Also, because hexamine is used in the phenolic resin curing agent, the ammonium ions that form as a result of hexamine decomposition dissolve out during operation of the fuel cell, leading to a decline in fuel cell output and reduced stability in sustained power generation.
In Patent Document 2, a separator that has a thickness of 0.2 mm or less and excellent mechanical strength is obtained. In Patent Document 3, a thin separator of good thickness accuracy having a thickness of from 0.2 to 0.6 mm and a thickness variation of within ±15 μm is obtained. However, because both of these separators use an ortho-cresol novolak-type epoxy resin having a high water uptake as the main ingredient of the binder resin, decreased performance and failure arise due to water uptake by the separator during sustained power generation by the fuel cell.
The separators in Patent Documents 4 and 5 make use of, as examples of the epoxy resin employed, biphenyl novolak-type epoxy resins (phenol aralkyl-type epoxy resins with a biphenylene skeleton) having a low water uptake. Biphenyl novolak-type epoxy resins have a low water uptake because the concentration of polar groups that form during the curing reaction is low. On the other hand, because the crosslink density is low, they have a poor heat resistance, making it necessary to select a resin having a high molecular weight. However, high-molecular-weight resins have a high melt viscosity, and so the flowability of the composition during molding is poor, resulting in a large variation in thickness and a high initial contact resistance.
In Patent Document 4, a high para-novolak-type phenolic resin is used as the curing agent. Because high-para novolak-type phenolic resins have a low crystallinity, the melt viscosity is high and the flowability of the composition during molding is poor, resulting in a large thickness variation and a high initial contact resistance. In Patent Document 5, slow-reacting 2,3-dihydro-1H-pyrrolo(1,2-a)benzimidazole is used as the curing accelerator, and so formation takes a full 2 minutes, which is a problem in terms of productivity.